Not applicable.
Not applicable.
The field of the invention is motor controllers and more specifically a method and apparatus for limiting current in an open loop adjustable frequency motor drive at low operating frequencies.
Induction motors have broad application in industry. An induction motor system typically includes a driver or controller, a power conversion configuration and an induction motor itself. The power conversion configuration generally receives power via supply lines and converts the received power into a form that can be provided to the motor thereby causing a motor rotor to rotate. The conversion configuration typically includes a plurality of semiconductor switching devices that link the supply lines to motor terminals and, based on switch turn on and turn off cycles, provide power to the motor phases linked thereto.
One common type of motor is a three-phase induction motor that includes a stator and a rotor. The stator typically forms a cylindrical stator cavity. One common rotor design includes a xe2x80x9csquirrel cage windingxe2x80x9d in which axial conductive rotor bars are connected at either end by shorting rings to form a generally cylindrical structure. The rotor is mounted in the stator cavity for rotation about a rotor axis. The stator windings are linked to three separate phases of the converter configuration to receive currents therefrom. The stator currents are controlled so that their combined effect is to generate a magnetic stator field that rotates about the stator cavity. The rotating stator field flux cuts across the conductive rotor bars and induces (hence the label xe2x80x9cinductance motorxe2x80x9d) cyclic current flows through the bars and across the shorting rings. The cyclic rotor bar current flows in turn produce a rotor field. Interaction (e.g., pulling or pushing action) between the rotor field and the stator field causes the rotor to rotate.
By using induced rotor current to generate the rotor field, the need for slip rings or brushes (i.e., wearable mechanical components) is eliminated which renders induction type motors relatively maintenance-free and reduces overall costs associated with motor design. Among other reasons, relatively limited costs have made inductance motors preferred for many applications throughout industry.
To a first approximation the torque (i.e., rotational force on the rotor) and speed of an induction motor may be controlled by changing the frequency of the driving voltage and thus the angular rate of the rotating stator field. Generally, for a given torque, increasing the stator field rate will increase the rotor speed (which generally follows the stator field). Alternatively, for a given rotor speed, increasing the frequency of the stator field will increase the torque by increasing the slip, that is, the difference in speed between the rotor and the stator field. An increase in slip increases the rate at which flux lines are cut by the rotor bars thereby increasing the rotor-generated field and thus the force or torque between the rotor field and stator field.
Referring to FIG. 10, the rotating phasor 13 of the stator magneto motive force (xe2x80x9cmmfxe2x80x9d) will generally form some angle xcex1 with respect to the phasor of rotor flux 19. The torque generated by the motor is proportional to the magnitudes of these phasors 13 and 19 but also is a function of their angle xcex1. The maximum torque is produced when phasors 14 and 18 are at right angles to each other (e.g., xcex1=90xc2x0) whereas zero torque is produced if these phasors are aligned (e.g., xcex1=0xc2x0). Phasor 13 may, therefore, be usefully decomposed into a torque producing component 15 perpendicular to the phasor 19 and a flux component 17 parallel to rotor flux phasor 18.
These two components 15 and 17 of the stator mmf are proportional, respectively, to two stator currents iqe, a torque producing current, and ide, a flux producing current, which may be represented by orthogonal vectors in a rotating or synchronous reference frame of the stator flux having slowly varying magnitudes. The subscript xe2x80x9cexe2x80x9d is used herein to indicate that a particular quantity is in the rotating or synchronous frame of stator flux.
Accordingly, in controlling an induction motor, it is generally desired to control not only the frequency of the applied voltage (hence the speed of the rotation of the stator flux phasor 13) but also the phase of the applied voltage relative to the current flow and hence the division of the currents through the stator windings into the iqe and ide components. Control strategies that attempt to independently control currents iqe and ide are generally termed field oriented control (FOC) strategies.
The production of any given set of currents iqe and ide requires that the stator be excited with voltages Vqe and Vde as follows:
Vqe=(Rs)(iqe)+(2xcfx80fe)(xcexrated)xe2x80x83xe2x80x83Eq. 1
Vde=(Rs)(ide)xe2x80x83xe2x80x83Eq. 2
where
Rs=stator resistance;
iqe, ide=synchronous motor currents aligned with the d and q-axis typically reflecting motor load and no load currents, respectively;
fe=electrical field frequency in Hertz; and
xcexrated=stator flux linkage=motor nameplate voltage/motor nameplate frequency (in Hertz).
The first terms on the right hand sides of each of Equations 1 and 2 are referred to as the stator resistive voltage drops. As the labels imply, the resistive voltage drops Rsiqe and Rside correspond to components of the voltage provided at a stator winding terminal that are dissipated by the stator winding resistance Rs. Because the resistive drops are provided to boost the commanded voltages and, in effect, overcome the stator resistance Rs, the resistive drops are often referred to as xe2x80x9cvoltage boostxe2x80x9d terms. The second term 2xcfx80fexcexrated on the right hand side of Equation 1 is referred to generally as a reactive voltage drop and, as its label implies, corresponds to the component of the voltage provided at the stator winding terminal that causes inductance or interaction between the stator and the rotor.
Equations 1 and 2 above are the fundamental command equations employed by most voltage/frequency controllers. To implement Equations 1 and 2, the controller has to be provided with several of the terms in each of Equations 1 and 2.
In order to minimize costs, often controller/converter configurations are designed to be useable for many different purposes (i.e., to drive many different load types). For instance, one controller/converter configuration may be capable of driving any of several differently sized three phase motors where the motors have different operating characteristics. Thus, when designing controller/converters, manufacturers typically do not know exact characteristics of loads that will be linked to and driven by the controller/converters and, therefore, some controller operating parameters have to be set by customers after system configuration is completed.
The rated flux xcexrated can be determined by dividing a name plate motor voltage by a nameplate frequency values while the stator winding resistance Rs is typically determined by performing a commissioning procedure (e.g., see U.S. Pat. No. 5,502,360 for a commissioning procedure to determine Rs). The d-axis current ide may be determined in any of several different ways including use of a look-up table that correlates d-axis current with various motor parameters or by performing some type of commissioning procedure. Each of the d-axis current ide, the stator resistance Rs and the rated flux xcexrated are stored in a controller memory for use during motor operation. The d-axis current ide typically is not adjusted during motor operation and therefore the d-axis voltage Vde is set upon commissioning.
In addition to the components described above, most controllers also include some type of feedback mechanism to ensure that an associated load (e.g., motor) operates in a desired fashion. To this end, typical feedback loops include a rotor speed feedback and d and q-axis current feedbacks. The feedback signals are generally compared to commanded signals and, where errors occur, the commanded signals are modified to force the load toward desired operating characteristics. For instance, where a feedback rotor speed is less than a commanded rotor speed, the rotor speed error can be used to command a higher electrical frequency thereby increasing slip and torque on the rotor and causing the rotor speed to increase by a percentage of the increase in the electrical frequency.
To implement Equations 1 and 2, after rated flux xcexrated, stator resistance Rs and d-axis current ide values are identified and stored in the motor controller memory, the controller receives a rotor speed command that indicates a desired motor rotor rotational speed. In addition, d and q-axis feedback currents idef and iqef are provided to the controller. The controller uses the commanded frequency and the feedback currents to generate suitable d and q-axis voltages Vde and Vqe, respectively by solving Equations 1 and 2 above. Thereafter, the controller converts the d and q-axis voltages Vde and Vqe into three phase voltage commands to drive converter switches.
As with all electronic components, the switching devices that comprise the converter configuration are designed to operate within specific rated current operating ranges and will be damaged or may operate in unintended ways when driven outside the rated current ranges. Unfortunately, during induction motor operation, conditions have been known to occur that cause controllers to demand current levels outside rated ranges. For instance, when a load is increased, the load will generally slow the rotation of a motor rotor which causes a difference between a commanded frequency and an actual frequency. The frequency difference or error causes the controller to step up the commanded voltage thereby, referring again to FIG. 1, increasing the q-axis current iqe. At high frequencies where the reactive drop is ten or more times the resistive drop, a reactive drop adjustment (e.g., fe adjustment) appreciably affects commanded voltage Vqe while at a low frequency where the reactive drop may be one-fifth or less of the resistive drop, a reactive drop adjustment may not be capable of avoiding a current trip. At some point, as the load is increased, the q-axis current iqe exceeds the high end of a rated switch current range and switch damage or malfunction may occur.
To avoid switch damage/malfunction, most controllers now include a xe2x80x9ccurrent trippingxe2x80x9d function wherein, when measured switch currents exceed the high end of a rated switch range, the control system trips and, in effect, cuts off current to the converter and load thereby protecting the converter switching devices. While tripping is clearly preferred to switch damage, tripping hinders system productivity and is to be avoided whenever possible.
To minimize current tripping, most controllers now include some type of current limiting feature. One common current limiting scheme reduces the commanded electrical frequency fe when the upper end of the rated switch current range is exceeded. Referring again to Equation 1, when frequency fe is reduced, the commanded q-axis voltage Vqe is reduced which in turn reduces the resulting q-axis current iqe.
Frequency reducing schemes work well at relatively high frequencies and poorly at low frequencies. This frequency based effectiveness difference is due to the fact that the commanded voltage splits between the resistive drop component Rsiqe and the reactive component 2xcfx80fexcexrated and the ratio of resistive to reactive drops is in great part based on frequency fe. For example, at high frequencies (e.g., a name plate frequency) reactive drop component 2xcfx80fexcexrated may be ten or more times resistive drop component Rsiqe while at low frequencies the reactive drop may be one-fifth or less of the resistive drop. At high frequencies where the reactive drop is ten or more times the resistive drop, a reactive drop adjustment (e.g., fe adjustment) appreciably affects commanded voltage Vqe while at a low frequency where the reactive drop may be a fraction of the resistive drop, a reactive drop adjustment may not be capable of avoiding a current trip. Other sources of error that can cause positive current feedback are also contemplated including imperfect switching characteristics that result in unexpected winding current levels, reflected waves caused by long power supply cables, etc.
Prior known solutions to the current tripping problem at low operating frequencies simply stepped the commanded voltage Vqe to some level less than the voltage boost level Rsiqe and therefore resulted in sudden, unintended and undesirable changes in output torque to the load.
Thus, there is a need for an inexpensive method and/or apparatus that can smoothly control system current levels so as to avoid current trip conditions without causing undesirable torque pulsations at low operating frequencies.
It has been recognized that the low frequency current tripping problem described above can be overcome by providing a voltage boost limiting mechanism that will hold the voltage boost level below a level that will cause a current limit condition. By implementing such a limiting scheme in conjunction with a frequency based current limiting scheme at higher frequencies, virtually all current tripping conditions, independent of frequency and independent of the source of excessive current, can be eliminated and overall smother system operation results.
To this end, the invention includes a method for use with an induction machine system including a controller and d and q-axis current feedback loops, the controller receiving a frequency command signal and generating d and q-axis voltage command signals, the method for limiting load current to a level below a limit current at low operating frequencies. The method comprises the steps of identifying an operating frequency, where the operating frequency is below a low threshold value: comparing a feedback current to the limit current and where the feedback current exceeds the limit current, reducing the q-axis voltage command value.
In one embodiment the step of comparing a feedback current includes comparing a q-axis feedback current to a maximum q-axis feedback current. In a more specific embodiment the method further includes the step of mathematically combining the limit current and a d-axis feedback current to identify the maximum q-axis feedback current. Here the step of mathematically combining may include taking the square root of the difference of the squares of the limit current and the d-axis current. Furthermore, the step of comparing a q-axis feedback current to a maximum q-axis feedback current may include subtracting the absolute value of the feedback current from the maximum q-axis feedback current to generate a difference value and the step of reducing includes reducing the q-axis voltage command value when the difference value is negative. Even more specifically, the controller may generate a nominal voltage boost value by multiplying a stator resistance value and the q-axis feedback current and the step of reducing may include multiplying the sign of the q-axis feedback current and the difference value to generate a signed difference value and mathematically combining the signed difference value and the nominal boost voltage to generate a limited boost voltage. Here the step of limiting further may include the step of mathematically combining the operating frequency and the limited boost value to generate the q-axis voltage command value. The threshold value may be less than one percent of a nameplate frequency for the induction machine.
In another embodiment the step of comparing a feedback current includes mathematically combining the d-axis and q-axis feedback currents to generate an instantaneous stator current and wherein the step of comparing includes comparing the instantaneous stator current to the limit current. Here, the step of mathematically combining may include taking the square root of the sum of the squares of the d-axis current and the q-axis current to generate the instantaneous stator current. Still more specifically, the step of comparing may include subtracting the instantaneous stator current from the current limit to generate a difference value and wherein the step of reducing includes reducing the q-axis voltage command value when the difference value is negative. Here a nominal voltage boost may be provided by a controller user and the step of reducing may include mathematically combining the difference value and the nominal boost voltage to generate a limited boost voltage. More specifically the step of limiting may further include the step of mathematically combining the operating frequency and the limited boost value to generate the q-axis voltage command value.
The invention also includes a method for use with an induction machine system including a controller and d and q-axis current feedback loops, the controller receiving a frequency command signal and generating d and q-axis voltage command signals, the method for limiting load current to a level below a limit current at low operating frequencies. Here the method comprises the steps of identifying an operating frequency, where the operating frequency is below a low threshold value: mathematically combining the d-axis feedback current and the limit current to generate a maximum q-axis feedback current; comparing the q-axis feedback current the maximum q-axis current and, where the q-axis feedback current exceeds the maximum q-axis current, reducing the q-axis voltage command value. In one aspect the step of mathematically combining includes taking the square root of the difference of the squares of the limit current and the d-axis current.
The invention also includes an apparatus for use with an induction machine system including a controller and d and q-axis current feedback loops, the controller receiving a frequency command signal and generating d and q-axis voltage command signals, the apparatus for limiting load current to a level below a limit current at low operating frequencies. The apparatus comprises a processor running a pulse sequencing program to perform the steps of: identifying an operating frequency, where the operating frequency is below a low threshold value, comparing a feedback current to the limit current and where the feedback current exceeds the limit current, reducing the q-axis voltage command value.